Hi,
Go to the site http://sjp.pl/slownik/ort/
There u will find the latest dictionary with .zip extension (something like: sjp-myspell-pl-<date>.zip) - download it and extract. After extracting there will be another archive pl_PL.zip. Extract pl_PL.aff and pl_PL.dic too.
Move both files to: ...\The Bat!\Dictionaries folder.
Close The Bat! and launch again. When composing new message choose from bottom bar "'Polski". Once chosen should be remembered as default.